Protein degradation provides an endogenous supply of amino acid indispensable for maintaining protein synthesis and cell survival in the absence of exogenous sources. It also serves a scavenging function by removing potentially deleterious abnormal proteins. We propose to use these as the basis for the selection of temperature-sensitive Chinese hamster ovary (CHO) cell mutants defective in protein degradation. Two of our selection methods are aimed at killing cells able to synthesize protein by providing them with toxic levels of radioactive amino acids or analogues. In two other procedures we plan to use replica plating to detect mutants on the basis of their sensitivity to amino acid starvation at high temperature. Mutants will be characterized and for their ability to degrade four classes of proteins: slowly and rapidly turning over endogenous proteins, abnormal proteins, and exogenous proteins. Also, as an adjunct to this investigation, we will extend these studies to normal human fibroblasts and those from patients with progeria (premature aging) at various stages in their proliferative life-span in culture. This system has been taken as a model for the normal cellular aging process. The use of genetic probes to study the mechanism and regulation of protein degradation should add significantly to knowledge about this process, which is at present poorly understood.